1. Field of the Invention
The present invention relates to a reflection-type hologram scale and an optical displacement measuring apparatus therewith.
2. Description of the Related Art
As conventional ultra-high-performance optical displacement measuring apparatuses, a system that detects the variation of an interference state corresponding to the movement of a scale that is a diffraction grating is known. As an example of a diffraction grating scale for use with such an optical displacement measuring apparatus, a hologram scale using a holographic grating is known.
Generally, there are two types of diffraction gratings--transmission type and reflection type. In a scale having the transmission type diffraction grating, a light source and a detector are disposed on both sides of the scale. In a scale having the reflection type diffraction grating, a light source and a detector are disposed on the same side of the scale. It is said that while the reflection type scale is suitable for a compact apparatus, the transmission type scale is suitable for a high-performance apparatus.
A reflection-type hologram scale having a transmission-type holographic grating and a reflection film has been proposed (see Japanese Patent Laid-Open Publication Nos. 5-232318 and 6-300520). For example, in the related art reference disclosed in Japanese Patent Laid-Open Publication No. 5-232318, (1) a technology for structuring a reflection-type hologram scale by adhering a scale substrate having a holographic grating and a protection substrate having a reflection film with adhesive agent (see FIG. 5), and (2) a technology for structuring a reflection-type hologram scale by forming a reflection film on a scale substrate, directly forming a holographic grating on the reflection film, and adhering a protection substrate on the surface of the holographic grating with adhesive agent.
In the case of (1) reflection-type hologram scale, since an adhesive agent layer is disposed between the holographic grating and the reflection film, unnecessary diffraction light is superimposed as noise. In other words, as shown in FIG. 5, when one incident coherent light beam enters a holographic grating, a 0-th order light component and a 1-st order diffraction light component are reflected by the reflection film through different optical paths. These reflected light components enters the holographic grating. The holographic grating diffracts these light components. Thus, as shown in FIG. 5, two 1-st order diffraction light components are obtained.
When the thickness of the adhesive agent layer is smaller than the diameter of the incident light beam, the two diffraction light components overlap and interfere with each other. Thus, the two diffraction light components are detected as one light beam. If the thickness of the adhesive agent layer is not equal in the longitudinal direction of the scale, a phase difference corresponding to the thickness of the adhesive agent layer takes place between the two diffraction light components. Corresponding to the phase difference, the resultant light beam varies in the interference light intensity and phase. Thus, when the reflection-type hologram scale is used for a displacement measuring apparatus, due to a variation of the signal intensity and a phase variation regardless of the grating constant, a displacement read error will take place.
To prevent such a displacement read error, it is necessary to suppress the deviation of the thickness of the adhesive agent layer against the wavelength of the incident light or to electrically compensate a detected signal. However, it is practically difficult to perform these methods. In the reflection-type hologram scale of type (2), since a holographic grating contacts a reflection film, such a problem does not take place. However, when a holographic grating is formed on a reflection film, another problem will take place. As shown in FIG. 6, a holographic grating is formed by recording interference fringes of two plane waves A and B on a hologram photosensitive layer. However, when a reflection film is disposed as a base layer of the hologram photosensitive layer, as shown in FIG. 7, interference fringes of the plane wave A and its reflection wave A' take place. The interference fringes are also recorded in the direction of the thickness of the hologram photosensitive layer. In addition, interference fringes of the reflection waves of the plane waves A and B also take place.
In other words, in the reflection-type hologram scale of type (2), along with the interference fringes of the plane waves A and B that are necessary for forming a hologram, unnecessary interference fringes take place. The unnecessary interference fringes are recorded on the hologram photosensitive layer. The unnecessary interference fringes are (a) the interference fringes of the incident light of the plane wave A and its reflection light, (b) the interference fringes of the incident light of the plane wave B and its reflection light, (c) the interference fringes of the reflection light of the plane wave A and the reflection light of the plane wave B, (d) the interference fringes of the reflection light of the plane wave A and the incident light of the plane wave B, and (e) the interference fringes of the reflection light of the plane wave B and the incident light of the plane wave A. These unnecessary interference fringes cause the diffraction efficiency to lower and fluctuate. Thus, the displacement of a measurement object cannot be accurately read.